This invention relates to injection molding and, in particular, to a sprue bushing assembly having an automatically actuated gate that opens and closes the sprue hole in response to the opening and closing of a pair of mold members.
In most injection molding machines, flowable material such as plastic is delivered from a plasticizer to a sprue bushing located in the mold by means of a nozzle that is coupled to the bushing. The material is heated in the plasticizer and is then driven by means of a screw piston under pressure into the sprue hole of the bushing from where it is fed by means of a series of runners into one or more mold cavities. The heated material is packed into the cavities to insure that they are completely filled with material and that the molded part does not contain airholes or voids.
Generally, the mold consists of a stationary member and a movable member that coact so that the mold cavities may be opened and closed along a commonly shared parting line.
When space allows, the nozzle of the plasticizer may be equipped with a control valve which operates to regulate the flow of heated material delivered into the sprue bushing. This type of control is desirable and oftentimes necessary to prevent excessive molding material from dripping or "drooling" from the sprue hole exit during the cooling and parting of the mold members. When the material is permitted to drool, the excess material forms strings that can impede the flow of later introduced material through the runners. The strings can also become trapped between the mold member preventing the mold from properly closing and causing damage to the mold members. The strings can also be drawn into the mold cavities where they are frozen into the later molded part thereby misforming the part and rendering it unusable for its intended purpose. Machine operators sometimes see that a drip or string has formed when the mold is parted but being heated preparatory to the next molding cycle. The operator will attempt to clear the excess material by hand before the mold closes. This, of course, presents a serious hazard in that the hand can be burned or even entangled in the moving parts of the equipment. When nozzle control valves cannot be used, various temperature adjusting techniques are employed in an effort to prevent or minimize drooling. Here the injection temperature of the material is reduced incorrectly to a level where the viscosity of the material becomes considerably less than the optimum temperature needed to ideally fill the mold. Although lowering the viscosity of the material tends to minimize drooling, the reduction in injection temperatures also leads to ineffective packing of the mold cavity and the creation of unwanted voids in the finished product.